Why would someone choose to be trans?

A lot of times, choice doesn't enter into the picture. Who they are simply doesn't align with their body and their upbringing. So they change things in order to make their lives fit them a bit better.

When choice is involved, it isn't as cut and dry as people might think. For instance, a person could have been raised to hate their own gender or suffer a lot of abuse, and thus they reject a part of themselves. Or they may suffer extreme fetishes, self-stigmatized gayness, sexist or "reversed sexist" beliefs, or simply don't fit in. They may feel that they don't fit their expected gender role.

And yes, there are likely those who live somewhat as the other gender to get attention, land roles in adult productions, find certain types of work, or simply make a political or social point. Sometimes it is a manner of relieving stress.